Quirks in STL string class

Today I found a pretty annoyance when using STL string class. The find function returns the index if it find the string, and string::npos if it does not. For example,

std::string str("joeliscrazy");
index = str.find("joes");

The find returns a type of string::size_type, which is size_t in most implementations. Now since size_type is unsigned, you no longer can write the following statement

if ( index < 0 ) { }

Unless you define index as a signed type, such as int.

Other string libraries (java, .Net and MFC) use signed type as index, thus avoid this problem. Since most modern platforms provide 32-bit integer, who on the earth require a unsigned int as string index?